This invention relates to a hydraulic pressure control valve for use in hydraulic braking system of a vehicle.
The hydraulic pressure control valves in vehicle braking system control the braking pressure supplied to rear wheel brakes with respect to the braking pressure supplied to front wheel brakes thereby preventing so-called locking phenomenon on rear wheels. Usually, the hydraulic pressure control valve actuates when the inlet pressure (which usually corresponds to the pressure supplied to front wheel brakes) increases to a predetermined level (a cut-off pressure) and, thereafter, the rate of increase in the outlet pressure (which is supplied to rear wheel brakes) is decreased with respect to the inlet pressure. For changing the cut-off pressure in response to the loading condition of the vehicle, there has been proposed to provide an inertia valve which is actuated when the deceleration of the vehicle being braked exceeds a predetermined level and, thereafter, the control valve controls the pressure supplied to rear wheel brakes with respect to the pressure supplied to front wheel brakes.
When the hydraulic braking system comprises two independent circuits arranged in so-called X-piping wherein one circuit is connected to the left front wheel and the right rear wheel and the other circuit is connected to the right front wheel and the left rear wheel, there is a shortcoming that when a failure occurs in one of the circuits in which the inertia valve is provided, the control valve acts to cut off the pressure in the other circuit at a relatively low pressure, while, in such case, it is desired to increase the rear wheel braking pressure in the other circuit so as to compensate the failure in the one circuit. Such shortcoming can be solved by providing two inertia valves in respective circuits which complicates the construction and increases the size and is expensive.